Wheeler JD et al. (2016), Ontogenetic changes in larval swimming and orie...

ECB-ART-44706

J Exp Biol 2016 May 01;219Pt 9:1303-10. doi: 10.1242/jeb.129502.

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Ontogenetic changes in larval swimming and orientation of pre-competent sea urchin Arbacia punctulata in turbulence.

Wheeler JD , Chan KY , Anderson EJ , Mullineaux LS .

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Many marine organisms have complex life histories, having sessile adults and relying on the planktonic larvae for dispersal. Larvae swim and disperse in a complex fluid environment and the effect of ambient flow on larval behavior could in turn impact their survival and transport. However, to date, most studies on larvae-flow interactions have focused on competent larvae near settlement. We examined the importance of flow on early larval stages by studying how local flow and ontogeny influence swimming behavior in pre-competent larval sea urchins, Arbacia punctulata We exposed larval urchins to grid-stirred turbulence and recorded their behavior at two stages (4- and 6-armed plutei) in three turbulence regimes. Using particle image velocimetry to quantify and subtract local flow, we tested the hypothesis that larvae respond to turbulence by increasing swimming speed, and that the increase varies with ontogeny. Swimming speed increased with turbulence for both 4- and 6-armed larvae, but their responses differed in terms of vertical swimming velocity. 4-Armed larvae swam most strongly upward in the unforced flow regime, while 6-armed larvae swam most strongly upward in weakly forced flow. Increased turbulence intensity also decreased the relative time that larvae spent in their typical upright orientation. 6-Armed larvae were tilted more frequently in turbulence compared with 4-armed larvae. This observation suggests that as larvae increase in size and add pairs of arms, they are more likely to be passively re-oriented by moving water, rather than being stabilized (by mechanisms associated with increased mass), potentially leading to differential transport. The positive relationship between swimming speed and larval orientation angle suggests that there was also an active response to tilting in turbulence. Our results highlight the importance of turbulence to planktonic larvae, not just during settlement but also in earlier stages through morphology-flow interactions.

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Genes referenced: impact LOC100887844

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	Fig. 1. A comparison of the relative size and morphology of the 4- and 6-armed larva. Scale bars (lower right), 100 μm. PO, postoral arm; AL, anterolateral arm. Reported length scales are stomach length, mid-line body length and total length (where total length is defined as the distance from the base of the body to the oral hood, neglecting arm length).
	Fig. 2. Larval movement and orientation tracking. (A) Sample experimental image, where an in-focus larva (with arms clearly visible) is highlighted by the white dashed box. Smaller white specks are passive particles and larger diffuse spots are out of focus larvae. (B) Close-up of the highlighted larva, with overlaid particle image velocimetry (PIV) velocity field (white arrows) surrounding it. Passive particle intensity is dimmed for clarity. (C) Sample time series of an individual larva's vertical swimming velocity as it was tracked in the field of view. (D) Close-up of the highlighted larva, with overlaid Cartesian coordinate system and shaded range of angles (−25 to 25 deg from vertical) at which the larva was considered upright. (E) Sample time series of an individual larva's orientation angle from vertical as it was tracked in the field of view. The shaded region denotes the range of orientation angles at which the larva was considered upright.
	Fig. 3. Larval swimming speed and time spent in vertical orientation at different turbulence intensities. (A,B) Median larval swimming speed (Vs) with 95% confidence intervals, with respect to the turbulence regime, for 4-armed larvae (A) and 6-armed larvae (B). (C,D) Median larval vertical swimming velocity (ws) with 95% confidence intervals, with respect to the turbulence regime, for 4-armed larvae (C) and 6-armed larvae (D). (E,F) Mean proportion of time spent by larvae within ±25 deg of the vertical orientation with 95% confidence intervals, with respect to the turbulence regime for 4-armed larvae (E) and 6-armed larvae (F).
	Fig. 4. Correlation between swimming speed and orientation angle. (A,B) Track-averaged orientation angle (0 deg is vertical) versus track-averaged swimming speed (Vs; A) and track-averaged vertical swimming velocity (ws; B). Each point represents an individual 6-armed larva, using larvae from all turbulence regimes. (C–F) Probability distribution of track-averaged swimming speed in upright larvae (within ±25 deg of vertical; C) and tilted larvae (outside ±25 deg of vertical; D), and probability distribution of track-averaged vertical swimming velocity in upright larvae (E) and tilted larvae (F). Asterisks denote median speed or velocity for each distribution. Note that the x-axes for the tilted larvae (D and F) are reversed, so that they are mirror images of C and E, respectively.

References [+] :

Chan, Effects of ocean-acidification-induced morphological changes on larval swimming and feeding. 2011, Pubmed, Echinobase